The invention provides a process and an apparatus for conversion of silicon tetrachloride to trichlorosilane.
Trichlorosilane is used for production of poly-crystalline silicon.
Trichlorosilane is typically prepared in a fluidized bed process from metallurgical silicon and hydrogen chloride. In order to obtain high purity trichloro-silane, this is typically followed by a distillation. This also affords silicon tetrachloride as a by-product.
The majority of silicon tetrachloride is obtained in the course of deposition of polycrystalline silicon. Polycrystalline silicon is obtained, for example, by means of the Siemens process. This involves depositing silicon on heated thin rods in a reactor. The process gas used as the silicon-containing component is a halosilane such as trichlorosilane in the presence of hydrogen. The conversion of trichlorosilane (disproportionation) to deposited silicon gives rise to large amounts of silicon tetrachloride.
Silicon tetrachloride can be used, for example, to produce finely divided silica by reaction with hydrogen and oxygen at high temperatures in combustion chambers.
However, the use of greatest economic interest for silicon tetrachloride is conversion to trichlorosilane. This is accomplished by reaction of silicon tetra-chloride with hydrogen to give trichlorosilane and hydrogen chloride. This makes it possible to obtain trichlorosilane from the silicon tetrachloride by-product formed in the deposition, and to feed that trichlorosilane back to the deposition process, in order to obtain elemental silicon.
The conversion of silicon tetrachloride with hydrogen to give trichlorosilane typically takes place in a reactor at high temperatures, at at least 600° C., ideally at at least 850° C. (high-temperature conversion).
For reasons of energy saving, the reactants in the reaction (silicon tetrachloride and hydrogen) are typically heated with the aid of the hot offgases of the reactor (products and residues of the reactants, i.e. essentially trichlorosilane, hydrogen chloride, silicon tetrachloride and hydrogen).
The patent DE 30 24 320 C2 claims a corresponding apparatus for conversion of silicon tetrachloride to trichlorosilane using a heat exchanger unit. The heat exchanger unit may consist, for example, of a set of electrically unheated graphite tubes which serve as a gas outlet, around which fresh gas flows on the outside by the countercurrent principle.
Reactors for the hydrogenation of silicon tetrachloride with hydrogen must be able to withstand high temperatures and the corrosive nature of materials such as chlorosilanes and hydrogen chloride gas, which is formed during the hydrogenation process. Therefore, carbon-based materials, including carbon, graphite, carbon fiber composite materials and the like, are typically used within the reactor.
DE 195 02 550 A1 discloses a process for hydrogenation of silicon tetrachloride, where the process comprises contacting hydrogen gas and silicon tetrachloride at a temperature greater than 600° C. in a reactor comprising a pressurizable shell having located therein a reaction vessel forming a substantially closed inner chamber for contacting the hydrogen gas with silicon tetrachloride, wherein an outer chamber is arranged between the pressurizable shell and the reaction vessel, the outer chamber having located therein and adjacent to the shell a carbon or graphite insulation layer and between the insulation layer and the reaction vessel one or more heating elements, wherein a gas or a gaseous mixture having a chlorine to silicon molar ratio greater than 3.5 is fed to the outer chamber.
The gas fed to the outer chamber may be silicon tetrachloride or a mixture of silicon tetrachloride with trichlorosilane, dichlorosilane or chlorosilane. The gaseous mixture fed to the outer chamber may also comprise chlorine, hydrogen chloride or a mixture thereof, and one or more silanes selected from the group of silicon tetrachloride, trichlorosilane, dichlorosilane and chlorosilane.
At high temperatures, graphite reacts with hydrogen to give methane (=methanization). This leads to structural defects in the reactor and ultimately to reactor shutdowns and a reduction in service life. Since defective parts have to be replaced and new parts have to be installed, this is also associated with a considerable level of financial investment.
The methanization occurs especially in the heaters which come into direct contact with hydrogen and silicon tetrachloride. In addition, the countercurrent heat exchangers, especially in the range of relatively high temperatures, especially in the region of the offgases, can also be damaged by the reaction of hydrogen and graphite to give methane. Particularly heating elements manufactured from graphite exhibit the greatest propensity to corrosion, since hydrogen (mixed with silicon tetrachloride) meets very hot surfaces here. Damage in the heaters is highly likely to lead to a shutdown of the converter reactor, since the heaters are designed as resistance heaters.
Since graphite is pervious to hydrogen and silicon tetrachloride due to its porosity, hydrogen and silicon tetrachloride can diffuse from the reactant side to the product side in the heat exchanger. This reduces the selectivity of the overall process, since not all reactants are conducted completely through the converter.
The reactant stream which diffuses to the product side does not reach the reaction zone, the result of which is that no conversion of silicon tetrachloride to trichlorosilane takes place. In addition, the product stream coming from the reaction zone is diluted, which is disadvantageous.
If the procedure of DE 195 02 550 A1 is followed, the heating elements are not in contact with hot hydrogen. There can therefore be no corrosion in this region. Damaging effects of hydrogen gas outside the reaction zone can be avoided.
However, the tubes which form the inner zone and define the outer zone can be corroded by the incoming hydrogen, which leads to reactor shutdowns in the long term.
Moreover, it has been found that, in the presence of dichlorosilane, there is deposition of silicon in the two tubes. This leads to a deterioration in heat transfer. In order to compensate for this, the heating output has to be increased further, which would have an adverse effect on energy balance.
EP 2 008 969 A1 describes a reactor for conversion of silicon tetrachloride, in which protective gas is used to prevent any leaks of process gases which occur. This involves conducting argon into an outer vessel, while no argon is present within the reaction vessel. This protects the heaters from hydrogen and other process gases.
EP 2 000 434 A2 also proposes an apparatus for conversion of silicon tetrachloride, in which the heaters are arranged separately from the reaction space. The space between reaction zone and outer vessel is supplied with argon under elevated pressure, in order to prevent leaks to the outside. Here too, the heaters are not in direct contact with hot hydrogen. Corrosion of the heaters is thus avoided. However, the heat exchangers are exposed to the hydrogen without protection.
The disadvantage of these methods is that the greater distance of the heaters from the product stream results in a much higher heater temperature being required. This higher temperature results in damage to the electrical bushing. Moreover, it causes a higher level of complexity for insulation of the heating space in the outward direction from the pressurized shell. Furthermore, this need for additional insulation increases the diameter of the plant. The heating space also has to be provided with complex pressure regulation which ensures that the pressure in the heating space is always somewhat greater than in the product space, in order that the hydrogen in particular cannot penetrate into the heating space.
However, the pressure in the heating space must also not be too great because the product chamber, which is exposed to extreme temperatures due to the heaters, is otherwise additionally stressed with elevated pressure and can therefore be damaged. Moreover, the lack of heaters in the reaction space results in loss of heat exchanger surfaces, which have to be additionally introduced, making the reactor even larger.
If the boundary shell of the heating space is slightly damaged, for example by chemical attack, the gas in the heating space flows into the product space as a result of the elevated pressure. This firstly means a corresponding loss of the gas in the heating space. Secondly, the product gases are contaminated by the incoming gas, which can have an adverse effect, especially for the downstream processes.
In principle, it is also possible to introduce the power into the product space not via radiation, but rather via induction. The disadvantages of this variant are likewise the complex pressure regulation and the greater diameter required for the plant. Moreover, insulation would have to be introduced between product space and heating space in order to protect the induction coil in the heating space from impermissibly high temperatures. In addition, the induction coil would have to be cooled, which leads to heat losses and hence reduces the efficiency of the reactor.
The patent DE 10 2006 050 329 B3 describes a high-pressure process for conversion of silicon tetrachloride to trichlorosilane. The conversion has to be effected at a supercritical pressure of the reactant gases. The system pressure of the product stream is preferably kept within the subcritical range. This is achieved by a decompression downstream of the reaction zone. Here too, it is found, however, that the heating elements and heat exchangers are corroded by hydrogen.
In the prior art, efforts have been made to coat the graphite parts used with suitable materials in order to achieve the effect that hydrogen can react only to a reduced degree, if at all, with the surface of the parts.
DE 10 2005 046 703 A1 proposes, for example, coating the surface of the reaction chamber and the surface of the heating element in situ with silicon carbide before the hydrogenation of the chlorosilane, and thus reducing methanization of these components. This step of coating with silicon carbide takes place at a temperature of at least 1000° C.
Nevertheless, in the case of coated graphite parts too, methanization and associated corrosion are always still to be expected.
In addition, it is also necessary to coat the heat exchangers—especially the hottest parts—which means a not inconsiderable financial investment, especially since consumables are still involved. Manufacture of the heat exchangers completely from SiC would also be conceivable, but this firstly likewise means an enormous financial investment, and manufacture of relatively large components from SiC or comparable ceramic materials, as actually required in production plants, is secondly possible only with very great difficulty, if at all.
DE 10 2005 005 044 A1 describes a process for conversion of silicon tetrachloride to trichlorosilane, in which the cooling rate of the process gas in the heat exchanger is controlled. For the heat exchangers, preference is given to using materials such as silicon carbide, silicon nitride, quartz glass, graphite, or silicon carbide-coated graphite.
However, the construction complexity is comparatively high and the use of such heat exchangers is thus relatively expensive.
DE 11 2008 002 299 T5 proposes purifying “dirty” trichlorosilane comprising dichlorosilane from crude trichlorosilane production, and then reacting it with silicon tetrachloride to give trichlorosilane.
If dichlorosilane is used as an additional component of the reactants in the conversion, however, additional continuous coating of the graphite parts like the heat exchanger is observed. This coating, which grows with time, alters the heat transfer and therefore has adverse effects on the energy demand. Moreover, the coating has adverse effects on the structural integrity of the components, especially the heating elements.
As a result of the dichlorosilane deposition reaction, which already takes place before entry of the dichlorosilane into the actual reaction space of the reactor, only a small amount of dichlorosilane remains to react with silicon tetrachloride. The increase in trichlorosilane yield is therefore in reality much lower than first expected.
This problem of dichlorosilane deposition exists in all processes and reactors known in the prior art when dichlorosilane is present in the reactant stream.
This problem gave rise to the objective of the present invention, that of providing a process and an apparatus suitable for performance of the process, which prevents methanization of components and, in the case of use of dichlorosilane, prevents deposition on the components and simultaneously increases the yield of trichlorosilane.